Boil potatoes until tender; cool slightly. Preheat oven to 400° F. Cut each potato in half. Scoop out centers*, leaving 1/4 inch shell. Mash the potato pulp; reserve. Combine butter, cheese, pastrami, milk and potato pulp until smooth and blended. Refill potato halves with mixture. Bake 20-22 minutes or until hot through and cheese is melted. Mix together mayonnaise and mustard. Top each potato with small dollop of mixture and garnish with dill, if desired.